Supercharging a diesel engine which powers a large commercial vehicle such as a truck or bus can improve engine/vehicle fuel economy and performance. A turbocharger is commonly used for supercharging such an engine. In the United States, governmental regulations also require that new vehicles comply with applicable tailpipe emission standards. Externally cooled, engine exhaust recirculation (commonly called EGR) is an effective technology for reducing oxides of nitrogen (NOx) in tailpipe emissions and may be useful in qualifying an engine design for compliance with certain tailpipe emission requirements.
While increasing EGR rates beyond present-day levels can further reduce NOx in tailpipe emissions, it appears that more devices would have to be added to a base diesel engine to accomplish that. The addition of such devices to a base engine may also impact other aspects of engine/vehicle operation such as engine/vehicle performance, durability, fuel economy, and/or manufacturing cost targets.
For example, it is known that moderate increases in EGR rates from typical present-day levels can be achieved by using one or more additional control valves, such as an intake throttle valve, for managing flow into the engine's cylinders. However, throttling the intake flow reduces engine efficiency, and the inclusion of additional components like an intake throttle valve may impair the ability to achieve EGR rate increases which are greater than moderate rates. Even if more than moderate increases in EGR rates can be achieved over a large portion of an engine operating map, significant engine pumping losses may occur in some regions of the map where the boosting system, i.e. the turbocharger, is operating with relatively poorer efficiency than in other regions.
EGR rate is affected by pressure in an exhaust manifold, i.e. by exhaust back-pressure. Control of exhaust back-pressure is an element of an engine control strategy because exhaust back-pressure can affect engine/vehicle performance, fuel economy, tailpipe emissions, and engine components including components in intake and exhaust systems. In a turbocharged diesel engine, a turbocharger can be used to control exhaust back-pressure.
Commercially available turbochargers have either single or multiple stages. Two types of turbochargers are wastegate turbochargers and variable geometry turbochargers (VGT's). In a two-stage wastegate turbocharger, a wastegate shunts a high-pressure turbine which is downstream of an exhaust manifold. The wastegate is in essence a valve that is controlled to selectively shunt engine exhaust around the high-pressure turbine.
When the wastegate is closed, all exhaust coming from the exhaust manifold, less any which may be recirculated as EGR in a high-pressure external EGR system, operates the high-pressure turbine. Increasingly opening the wastegate increasingly shunts engine exhaust around the high-pressure turbine. The extent to which the wastegate shunts engine exhaust affects not only exhaust back-pressure but also pressure developed in an engine intake manifold by the high-pressure turbine's operation of a high-pressure compressor in the intake system.
The difference between pressure in the intake manifold and pressure in the exhaust manifold affects both engine efficiency and external EGR flow.
Even if engine modifications like those mentioned earlier could be implemented successfully in an engine to create proper air/EGR charge over the entire engine speed-load domain for a range of EGR rates more extensive than typical present-day ranges, while at the same time meeting engine/vehicle performance and fuel economy targets for all engine operating conditions (e.g., EGR off, high altitude operation, cold start), the implementation is apt to add significant complexity, especially for a diesel engine which has a multi-stage turbocharger capable of developing high intake manifold pressure (i.e. boost).
Moreover, the engine modifications should not have significant adverse effects on other systems, such as exhaust after-treatment by processes such as selective catalytic reduction (SCR) and/or diesel particulate filter (DPF) which depend on proper exhaust temperature and composition.